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How can flow in the compressor of a jet engine go back to the inlet?
In aerodynamic lessons and books about jet engines, it is always said that there is a risk that air flow in the combustion chamber or compressor can go back to reverse direction, towards the inlet if something goes wrong, for instance a compressor error, surge.
What I don't understand here is that when we imagine the engine in flight, the engine cruises very fast into the air, which means that there is a remarkable amount of air rammed into the inlet towards the combustion chamber.
Even though the compressor doesn't work properly and push the air to the combustion chamber, there is still much air coming into the inlet and pushing the air in the compressor towards the combustion chamber. How can the air in the combustion chamber go back while another air volume is coming to it from the inlet side continuously?

Related: Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?, and What exactly is a compressor stall?

I used to have the opposite question, why don't the gases expanding in the combustion chamber force their way out the front all the time... But I think these answers clarify that question as well.

Note that the flow speed past the diffusor (which sits between the compressor and the combustion chamber) is very low in order to allow air and fuel to mix, the fuel to evaporate and the mixture to ignite. Little energy is needed to let the compressed air change direction and accelerate into the compressor.
Burning the fuel will heat up the air, so it expands. A lot. Now where should that added volume go? At the front the compressor pushes new air into the diffusor and at the back the turbine is blocking the flow, albeit less than the compressor.
When the compressor stalls, the pressure it creates drops, so the resistance which before prevented the hot air from expanding into the compressor is gone. Now the turbine will block the flow more and the hot air expands into the compressor.
While the ram pressure raises the pressure even before the compressor, the pressure in the combustion chamber is much higher because the compressor was working as intended until the surge occurred, raising the pressure far above the total pressure of the air.

The reason is the ram air mass flow entering the front of the compressor isn't that strong relative to the other forces happening within the engine. You have a system that depends on mass and inertia moving air from front to rear to keep the cycle going, and most of the mass and inertia packing air into the combustor, and forcing it to expand out the back, is generated by the compressor.
When the compressor stalls, the pressure and velocity of the air flow into the combustion chamber drops off sharply. The pressure in the combustion chamber from expanding gases, normally forced to expand rearward by the air piling in from the front, is able to expand both rearward and forward.
This creates an intense pressure wave of hot expanding gases forward, and so this blast of hot air from the combustion chamber overpowers the relatively weak ram air flow from the engine's forward movement, sometimes with still burning fuel in it making orange flames, out the front cowl.

If the compressor blades stall, the pressure inside the combustion chamber then "shows up" on the inlet side of the compressor stage. That pressure will block the incoming airflow which then spills out and around the inlet lip instead of entering the engine.


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The fuel shut-off valve was left in over night. Is it dangerous for the plane?
I forgot to pull the shut-off valve in my plane (PZL-110 Koliber 150) while I was trying to start an engine.  Will this have any effect on the plane?
It has a mixture supply knob and is a pull/push type. I have pushed in and, according to the checklists, it should stay on the pull position while the engine is not running. It is also a low-wing aeroplane.

What kind of airplane?

PZL Koliber 150. Like C172N with carburetor.

You need to explain more.  Was in OFF or out OFF?  Is it a push pull knob?  What does the POH say to do?  I have a low wing airplane with a carbureted Lycoming, and I never turn the fuel off unless I'm working on the fuel system. That is, the fuel is ON 24/7.

Sorry, i mean a mixture supply knob, it is pull/push knob. I have pushed in and according to the chcecklists it should stayed on pull position while engine is not running. It is also low wing aeroplane.

Please update the question with the details from your comments.

A mixture control left at full rich has no effect on a carbureted engine when it's not running. It's just a best practice to cater to, say, some situation where someone moves the prop by hand (and they don't bother to check the controls) and it could potentially put a fuel charge in a cylinder (if they flip the prop fast enough), and maybe somehow it could fire and kick the engine over (that generally doesn't happen on a cold engine - but maybe there is a hot mag that went undetected; lots of what-ifs).
Anyway, it's a procedural fault, but no big deal.

I think you forgot the most important part - Take your wet noodle lashings and never do it again! Or else! ;)

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Would aft stabilisers, that can move up and down, have any benefits?
I was looking at some aircraft as an inspiration for a project and found the X-02 Wyvern from the Ace Combat series and one feature that seemed interesting (let's ignore its Swiss Army knife wings) was the aft stabiliser's ability to change their angle.
Would there be any benefits to it if the airplane could change the angle dynamically, say from -35 to +35 degrees on the roll axis?

Could you turn that round and consider how much it might cost to let the aft stabiliser's change their angle?
Does that suggest the ability might be worth sqillions?
If you doubt there'd be any benefits to a plane changing the angle, say from -X to +X degrees on any axis, what benefit do you suggest any given, fixed angle would offer?

@RobbieGoodwin if there is a benefit, then the cost problem is, as they say, "just" engineering.

While clearly a disadvantage during take-off and landing, an anhedral tail would be rather helpful at supersonic speed.
Vertical tails pointing up produce a rolling moment which runs counter to the desired effect when starting a turn. If they point down, their roll contribution becomes helpful. However, fighters use their ailerons for starting turns, not their rudders, so this effect has little relevance in real life.
Next, consider the graph below which shows that the ventral fins of the F-104 Starfighter contributed 30% of directional stability at Mach 2. Tails pointing up at that speed are much less effective because air density above the fuselage is much lower than below at supersonic speed.

This means that rotating the vertical tails around the roll axis allows to make them smaller because they are more effective when it counts. However, so far the downside of mechanical complexity and added mass seem to have prevented this.

Yes, absolutely.  They are called elevons and can be found on many delta designs.
Elevons can function oppositely as ailerons, or together as an elevator.
The benefit is that, at very high speeds, having them closer to the fuselage on a strong structure reduces torque stress on the wings, which can warp if the aileron force is too great.
If one wished to lower the vertical stabilizers to a horizontal position, this would result in a loss of directional stability on the yaw axis.
Raising or lowering an aft horizontal surface to vertical, as with the XB-70, can have beneficial effects by increasing yaw stability, and increasing lift coefficient at supersonic speeds when lowered.
Raising the canards would result in a loss of yaw stability.  It may be better to bring them completely in line with the wings at higher speeds, as wing strakes, and to raise them up at lower speeds/higher Angle of Attack as stall inhibiting canards.

The OP asked about rotation around the roll axis, elevons rotate around the pitch axis.

He's talking about their function as stabilizing surfaces, where the thing morphs by the introduction of dihedral from rudderless horizontal tail only (top picture), into a V tail (bottom picture). An elevon function involves the surface moving on its pitch axis.

Please see edits.  Perhaps loss of yaw stability may help for fighters, but be ready for ... "you're gonna need a bigger computer".  The ability to raise or lower them would make for excellent spin recovery research.

Yes they have a benefit, as demonstrated by the F-16 all-flying-tail (the stabilators) for almost half a century.
Cropped image from the wiki
With all of the horizontal tail deflecting there is a huge amount of pitching moment created, great for manoeuvrability and tight turns. There is also a varying amount of passive aerodynamic pitch stability, no problem if the airframe does not require that anyway because of the artificial stability generated by the fly-by-wire inputs.
So yes, very beneficial, with structural and controllability issues being introduced which can be solved by active control engineering.

I didn't know the F-16 stabilators could move around the roll axis. I thought they just rotate around the pitch axis.

Pitch rotation is normal, but if you look at the attached pics of the wyvern, you see that they are able to shift in the roll axis, that is what i am curious when it comes to it's potencial mobility characteristics

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Why vacuum instead of pressure to power gyros?
Why are most air powered gyro instruments powered by vacuum rather than positive pressure?

There are piston aircraft that use positive pressure to power gyros.  The Beech Baron and Bonanza lines as well as the Piper Navajo line use pressure instead of vacuum. We're not talking about one-off airplanes here!  The Baron/Bonanza and Navajo are some of the most popular airplanes in their respective categories which, when summed, total many thousands of airplanes (and pressure pumps).
The turbines I've flown used ejector pumps, not mechanical pumps, to draw vacuum so I will assume that this class of plane is out of scope for the purposes of this question.
A technical answer to your question will require an aircraft system engineer (perhaps Peter?) to weigh in.  My guess is that vacuum pumps were installed to replace venturis.  Since systems (and instruments) were already designed to run vacuum, the pumps were installed to draw vacuum instead of apply pressure.  Why change what works?

I think you are on to something with the transition from Venturi hypothesis.

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Why are static ports external on a non pressurized aircraft?
Seems like the intruments in a non pressurized aircraft have access to the static pressure right where they are. It seems like external ports would be subject to more error being exposed, and lines that can be obstructed.
What benefit is having the external ports?

Because while it isn't pressurized it's still pretty well sealed. Most alternate static ports on GA aircraft are inside the cabin and have an associated error with them.

Remember Bernoulli? Lift is created on the wings because the air pressure decreases as the air moves faster. Same thing around the fuselage, but to a much lower extent. The air flowing around the fuselage creates a low pressure area so air in the cabin moves from inside the cabin to outside causing a lower pressure inside the cabin. Opening the cabin air vents can cause the pressure to be higher inside the cabin than outside.
Many general aviation airplanes that are used for IFR flying have two external ports that are placed well back along the fuselage. The  ports are usually positioned in areas where the boundary layer of air tends to be very stable, which lessens the effect of the static ports being hampered by turbulent flow and the Bernoulli effect.
Barry Schiff has an interesting explanation of how the pitot-static system works and shows how the placement of the static ports affects the reading of the instruments.

This answer has a nice graph of pressure differences around fuselage which shows there are only two or three suitable locations for the static ports. Might be worth copying here.

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Which aircraft type flies the most annual passenger-miles?
Which aircraft type flies the most annual passenger-miles?
I understand there are over 4,000 737s in service. However, there are over 1,300 777s in service.
These each carry more passengers, and presumably spend more time in cruise, where miles are clocked up fastest. Is it possible the 777 flies the most annual passenger-miles?

I'm going to attempt to partially self-answer my question in terms of seat-hours, using numbers from Boeing:
The post 'All in a day's work' tells us that in 2011 there were 'nearly 1,900 777 flights taking off every day' with an average flight length of six hours.   So I make that 11,400 daily flying hours for 777s in 2011.
The Boeing 737 Facts fact sheet tells us there in 2013 there were on average 2,000 737s in the air at any given time.   So I make that 24*2000= 48,000 daily flying hours for 737s in 2013.
If I make the fairly rough-and-ready estimate that a 737 has on average 150 seats, and a 777 330 seats, that gives us roughly 3.8 million daily seat-flying hours for a 777 in 2011, and roughly 7.2 million daily seat-flying hours for a 737.
I expect a 777 would spend more time in cruise, so the 777 flying hours would translate to more miles, but not enough to close the gap.


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Which airliners could survive an EMP pulse?
The incredibly bad, postapocalyptic tv series Revolution begins with a mysterious event that causes all electrical systems to fail all over the globe.  Even batteries don't work anymore.
In the first episode, after the "event" airplanes immediately start falling out of the sky in flat spins.
This had me thinking: which airplanes could survive an EMP-type event (FN1) that fries all electronics and renders electrical systems unusable?  This means that even RATs would not be a viable choice.
Piper cubs and other single-engine planes with purely mechanical linkages could still keep on gliding and land safely.
I don't think any large airliner currently flying could make it. Could even a B-17 or DC-10 make it with no powered electrical systems -- just on unpowered hydraulics alone?
Bonus question: if they did fall out of the sky, I'm assuming it would be in lawn dart mode and not in flat spins?
Footnote 1: While the apocalypse event in Revolution turns out to not be an EMP, you can assume it was something similar for the purpose of this question.

For what it's worth, the 727 had a manual reversion system for the control surfaces in case of hydraulic failure. The 747-100 and -200 required hydraulics, but the hydraulic valving was controlled by cabling, not electronics.

Air Force One was supposed to resist an EMP pulse, according to Donna Bullock

@Radu094 Technically speaking, Air Force One does not designate a given airframe, but any USAF aircraft with the President aboard. Usually, of course, it's one of the two 747-200s, tail numbers SAM28000 and SAM29000. The SAM stands for Special Air Mission as I recall.

@Terry They actually did get that right toward the end of the movie. "Liberty Two Four is changing call signs - Liberty Two Four is now Air Force One!"

I wonder how much an EMP would actually affect an airplane, flying or not.  They're basically faraday cages.

@Kevin I expect the radios would be able to couple the pulse to at least a few other sensitive systems (autopilot & FMC at the very least) - the antennas make nice penetrations in the skin's pulse shielding ability (as do the windows). We could just borrow someone's A-380 for a low-altitude flyby of the Navy's EMPRESS antenna and zap it to test though :)

One suggestion to make the question more clear: Since the event in Revolution had nothing to do with an EMP, it would be good to drop the reference to the tv show completely, or on he other hand, drop the reference to EMP if you want to know what happens when all electrical energy ceases to exist. The answers are different for the two scenarios and are muddying up the answers.

The S.M. Stirling novel "Dies the Fire" has a more realistic depiction of what would happen in such an "event". One of the main characters started out flying in an airplane when the "event" happened and had to glide to crash/land in a river.

Older airliners use hydraulics and cables attached to the pilot controls. Some RATs power the hydraulics directly as well so limited control is still possible then. The pilots would get a workout but the plane would safely touch down.
However planes with entirely fly-by-wire systems (no current airliner is completely fly-by-wire) will be completely uncontrollable apart from shifting weight and would glide to the ground. They are made stable so they won't just enter a spin and pancake into the ground.

"They are made stable so they won't just enter a spin and pancake into the ground." - does this mean they will default into a glide and land on their belly and not just lawn dart?

@RoboKaren yes, it's because the horizontal stabilizers will pitch the plane up when it speeds up and down when it slows down leading to a stable glide path.

A stable glide to earth sounds potentially survivable in a good number of cases, yeah?

@raptortech97 There are cases where light aircraft with an unconscious pilot ran out of fuel and basically landed themselves in a field, with the pilot walking away with only a few cuts and bruises. Such a nice outcome is by no means guaranteed, but if circumstances conspire to give the plane a nice flat landing area with no mountains or trees to crash into "just gliding into the ground" is eminently survivable.

All Airbus aircraft are demonstrated controllable with complete electrical failure. They would be pretty difficult to land if the electronics didn't at least partially recover (because the backup only allows controlling pitch and yaw and roll only via yaw-roll stability), but it would still be possible. Well, they probably don't count as "completely fly-by-wire" then; only some fighters do (that are fitted with ejection seats too) do.

Well the key to an airliner (or any other vehicle) surviving a real EMP would be having engines and flight control systems that don't require any kind of computer assistance. Hydraulic control systems powered directly by the engine and not relying on a computer to operate the valves would continue to function, as would cable-driven control systems. Simple electric motors may also continue to operate (larger motors tend to be EMP-resistant, in smaller motors there's a chance the EMP could burn out the windings or arc over and destroy the brushes) which means we can assume you'd still be able to pump fuel into the engines of most jet airliners.
Thinking along those lines I imagine the remaining DC-3 fleet would be almost entirely unfazed by an EMP (save for any retrofitted avionics, radios, etc. which would likely be fried). Similarly the Boeing 707 would probably not have a problem, nor would early-generation DC-9s, 727s, and 747s - you would lose any retrofitted flight management computers, but they are generally not essential to operation in the earlier-generation aircraft.
There are probably several other models still in service (or hanging around various boneyards) which are at least somewhat EMP-resistent.

If we assume a Revolution-style apocalypse where absolutely nothing using electricity works you're rather worse off: only aircraft which have cable-driven controls (or hydraulic backups with manual valve actuation & direct pressurization from the ram air turbine) would have functional flight controls. Thats a much smaller subset of the fleet these days, but the planes do exist - as Terry pointed out the 727 and 747 both have non-electric control systems available.
If we assume magnetos can still generate a spark the DC-3 other planes of its generation would be the clear winner: The engines would continue to run until stopped (the engine-driven fuel pumps would continue to feed the engine fuel). Aircraft like this could even possibly be restarted (using shotgun starters or pull starters, and a little help getting fuel to the engine until the mechanical fuel pump could take up the job).
If we assume magnetos would also stop working then at least you'd have flight controls and the ability to make a controlled emergency landing somewhere as you could in the other planes mentioned.

The answer depends on the strength of the EMP at the aircraft location. This obviously depends on the distance from origin and strength at source.
Aircraft do have some immunity from EMP, they don't just fall from the sky if you make sparks with a 12V car battery 35,000 feet below them.
Here's an extract from Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack.

EMP simulator HAGII-C testing a Boeing E-4 aircraft. (Wikipedia)
I've read that many modern FBW aircraft (including Airbus and Boeing) still have some fallback manual control of basic flight controls.
However if all your navigation and communication systems are fried and ATC is completely  dead, your chances of gliding to an essentially uncontrolled nearby airfield and making a good landing must be quite small. Especially at night, in the rain, with a new moon.

Having been in the Air Force and worked on bombers and fighters that can carry nuclear weapons all systems avionics and electronics are shielded from EMP. They are not “one time use” and we want them to return. We have divert bases that some may make it to for refuel and rearming Tierra Del Feugo is one such place. Away from strategic baes in the northern hemisphere. B-52s have been shielded and actually have a lead lined curtain that the pilots pull across the inside of the forward facing cockpit glass. Special grounding on the electrical systems for electronics and avionics direct the electrical pulse away from equipment and mechincal devises that have sensors. Normal passenger aircraft do not receive  such attention to detail and any communication equipment would be knocked out. However certain types of engines would remain in operation. What would affect our Air Craft the most is the radioactive blast wave if they were static on the Browns. The Enola Gay flew through the EMP and returned to base along with the second camera airplane, though a lot of film on it was ruined. The two aircraft that dropped bombs on Nagasaki also returned to base without incident.
Aircraft that dropped test bombs on Bikini Atoll never had any problems returning to base. We have developed weaponary that can deliver an EMP without a nuclear explosion, but what's in the works now are lasers


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Does the wifi uplink antenna affect aerodynamics or efficiency?
The wifi uplink antenna on airliners seems like a quite aerodynamically inefficient hump, just sticking up into the airflow above the main cabin. If it added even 0.1% more drag, I don't see how it would be financially viable.

(Photo by me, released to public domain)
How much does that big antenna drag? Why not either smooth it out forward and aft or build it into the body of the plane?

Not directly the point of your question, but it's not a WiFi antenna, it's a radome protecting the satcom antenna/RF amplifier used to transfer the traffic between the WiFi access points located within the cabin and Internet, via a satellite. The satcom antenna actually transfers both wireless and wired traffic on board, and voice as well as data.

Well, yes, if you want to be technical.

It seems an important detail. The antenna looks mechanically adjustable for tilt and electronically adjustable for pan. From the image the answer to your question is that locating the adjustable satcom aerial it into the body of the plane requires an alteration of the struts because of its size. That means also to update the airworthiness certificate. The WiFi access points antennas are much smaller and not mechanically adjustable don't have this problem..

@RoboKaren Since there are mostly technical people reading here, why don't you replace "wifi antenna" with something like "wifi service uplink antenna" or "satellite antenna for internet uplink" ?

One factor you didn't consider is that the entire airline might not be financially viable if it doesn't offer WiFi to business travelers on key routes, even if it increases drag. If you're negotiating a corporate contract for hundreds of seats for busy employees who frequently travel between offices, not having internet access is going to be a big disadvantage. As an example, United had WiFi on its business travel-heavy SFO/LAX-NYC transcon routes years and years before it started rolling it out to the rest of the fleet.

Any kind of antenna like that is creating an increase in the flat plate area of the aircraft and thus increasing the parasite drag on the airplane.  Fortunately newer antenna designs, such as the KU/KA band satellite internet systems can be arranged in a manner which greatly reduces the frontal area of the antenna.
The ideal solution, which will inevitably come about on future airliners as these systems increase in popularity with the travelling public, is the creation of an integral compartment on the dorsal side of the fuselage aft of the rear pressure bulkhead and fitted with a dielectric which is flush with the aerodynamic envelope, thereby eliminating parasite and interference drag as well as affecting airflow over the vertical fin.  However for earlier aircraft or those currently in service, this is not going to be possible for several reasons.
First, an installation like that would require major modifications to aircraft structure and pressure vessel of the fuselage and cost big, big bucks in order to get an STC or approval from the airline's engineering department.  Current designs are arranged to as they are to be minimally invasive; all that's required for a typical installation is the attachment of a series of brackets and longerons to the cabin frames which can then be attached to brackets outside of the aerodynamic envelope with a minimum series of fasteners penetrating the fuselage.  This arrangement is preferred by the airlines as their engineering departments and the FAA or other regulatory authority can sign off on the installs without requiring the OEM's involvement.
Secondly, many jets are not owned by the airlines themselves but are leased from financial brokerage firms and any kind of system which minimizes alterations to the airframe is preferable to major - and irreversible - alterations to the aircraft.
As for the economics of these systems, even if you did have an increase in fuel burn by 0.1% you could easily compensate for the loss if installed on a large enough aircraft.  Let's assume on a 747-400 flight from LAX to Narita Intl in Tokyo, you will typically burn about 290,000 lbs or about 44,000 gal of JET-A.  A 0.1% increase results in a mere 44 gal difference in fuel consumption at about $220.00 if you price the fuel at 5 USD/gal.  For a long haul fight like that, lets say that 25% of the passengers use this service at a cost of 44.00 USD each.  With a typ. 747 seating of 440 people, that's 100 passengers buying WiFi service totaling 4,400 USD in income for the carrier.  It's well worth it for them!

Great answer. I think you're widely optimistic about wifi utilization though. Of all the passengers, maybe half will use their laptop or tablet. Of those, maybe one fifth optimistically might buy the $40 internet pass. But half of that is maybe given up in licensing fees. So would be 440 * .5 * .20 * $40 * 0.5 = $880. But I guess that's still $836 in profit.

@RoboKaren If you only need to "save" 220 USD per flight, you can just shop around for a better deal on the champagne you serve in business class ;)

@alephzero, you could do that without installing a WiFi too and come out even better. No, to make sense, it is the WiFi that has to make up for the 220$ per flight.

Anything new attached to the outside of the airplane will change the aerodynamics and efficiency in someway; that is unavoidable.
The only question is if it will disrupt the airflow in a way that has a significant negative impact on the plane.
You'll recognize that the antenna is installed very close to the CG, so the new air resistance has virtually no pitching moment.
It is far from any control surface, so those all work the same.
The increased air resistance affects fuel economy a very tiny amount, but paying customers have enough preference for in-flight WiFi that its worth it.


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Does the ARCA hoverboard violate known limits on small-diameter electric fans?
The ARCA board is being hawked as achieving 430 pounds of lift from 36 electric fan motors for six minutes of 'flight.'

c.f. : http://www.theverge.com/2015/12/24/10663228/arcaspace-arcaboard-hoverboard-skateboard
To my uneducated eye, getting 12 pounds of static thrust from each of the small fan motors (430 pounds / 36 motors) seems unreasonable and perhaps impossible. The engineer says that it has 272 horsepower, so that would be 7.56 horsepower or 5.6kW per motor, which seems on the high side.
What are the a) theoretical and b) realistic amounts of thrust that one can assume from what looks like a 10-15cm diameter fan?

Thanks mins, you caught my math mistake on the horsepower.

The key is that the Arca board only works in ground effect. Therefore, the pressure difference over the area of the board is the key for calculating its power needs.
From the video I would estimate the size to be 0.6 x 1.2 m thanks to @mins more thorough research we know that the board measures 1.45 x 0.76 m for an area of 1.1 m². To lift both the occupant and the board itself it claims to create 430 pounds of force, equivalent to 195 kg of mass in Earth's gravity, so the pressure increase between top and bottom is 1740 N/m² or just 1.7% of atmospheric pressure. This is not much when you consider that a turbofan compressor stage can create a pressure differential of 1.7, albeit at much higher speeds.
The concept is basically the same as the Hiller flying platforms of the 1950s, the first of which was powered by two 44 HP engines. Since the intake geometry of the Arca board is - how should I say - less than optimum, it can not fly as high as the Hiller vehicles and I find the claimed 272 HP hard to believe and the high power draw is due to the low efficiency of 36 closely coupled blowers.

Hiller VZ-1 Pawnee (picture source)
To calculate the exact amount of power needed I would need to calculate the mass flow from below the board at the given pressure, which depends nonlinearly on altitude. Then I would need to calculate the power to produce this mass flow at the known pressure differential with 36 small blowers. Sorry, but even my Hoerner books do not cover this. What is certain is that the thrust from the 36 blowers is not nearly sufficient to lift the 195 kg when out of ground effect. The board only works at very little altitude when the outflow cross section around the board is restricted. This gives it an automatic altitude limit of a few centimeters and makes it self-stabilizing in pitch and yaw.
It takes some thick skin to watch the full length of this platitude-filled piece of marketing for an obvious gimmick. The camera angles were chosen to make it look fly higher than it actually can, and in the one shot where it does fly higher you cannot see who or what is standing on top. Anybody spending 20 grand on this is going to regret it quite soon.

EDIT:
With the answer provided by Bruce Abbott to @mins' question on Electrical Engineering SE the question can be solved. Bruce approaches the problem from the electrical side and links to a 120 mm blower which creates 74 N thrust and consumes 7 kW of power. He calculates 65 kg for the batteries and 36 kg for the blowers, for an estimated mass of the board of 110 kg. However, in combination the thrust from closely coupled blowers will certainly be lower than that of an isolated blower with ideal flow conditions.
Conclusion: Looks possible, no limits are busted, but there is some potential for further optimization and badly needed mass savings on the board.

Doesn't the Hiller flying platform gain some induced lift from the air traveling over the curved top surface of the air intake?

@RoboKaren Exactly right; the high flow speed creates suction. The Arca board neglects this, hence my "less than optimum" remark.

The Hiller flying platform gained some induced lift from the air traveling over the curved top surface of the air intake but the same curved surface righted up all the time the hoverboard as soon as the pilot wanted to travel in the forward direction. This is a serious drawback and one of the reasons the platform was abandoned as impractical. (see the explanations from here: http://www.rcgroups.com/forums/showpost.php?p=33605165&postcount=18)

Does the ARCA hoverboard violate known limits on small-diameter electric fans?

1) The max theoretical static thrust that can be obtained with an Electric Ducted Fan characterized by: Diameter = 120 mm and Power = 272 hp / 36 = 5.63 kW is:
(1.2 kg/m^3 x (5.63 kW)^2 x pi x (120 mm)^2/2)^(1/3) = 9.7 kgf
(I used the formula that gives the max possible static thrust as a function of power and the diameter of the propeller. The overall efficiency is considered 100%. For realistic efficiencies, smaller than 1, the Power is not 5.63 kW but 5.63 x efficiency)
As you see, 36 fans, that draw a total of 272 hp, can lift in theory 9.7 kg x 36 = 349 kg, well above the 192 kg of ArcaBoard (including the weight of the pilot). No limit is violated.
However, the configuration with 36, 120 mm diameter, rotors is bad because the same formula I used above says that a single 26 inch propeller powered by a 5.63 kW motor generates:
(1.2 kg/m^3 x (5.63 kW)^2 x pi x (26 inch)^2/2)^(1/3) = 30 kgf
In consequence, eight 5.63 kW motors, turning 26 inch propellers, will draw only 45 kW (60.4 hp) and lift 240 kg, more than the max mass of ArcaBoard in flight.
Small diameter propellers are simply inefficient for generating static thrust. This is the reason helicopters have large rotors. Hovering boards like ArcaBoard, also perfectly realizable, have no future because they waste an enormous amount of power.
2) A well designed and built electric hoverboard was made by C. A. Duru. It requires considerably less power than ArcaBoard and flies much better.
A comparison between the hoverboard of Catalin Alexandru Duru (see: the video) and that made by Dumitru Popescu from ARCA.

Video: C.A. Duru flying
Catalin Alexandru Duru's hoverboard

8 rotors, 4.5 kW each
Total Power: 36 kW = 48.3 hp
Payload: 1 person

Dumitru Popescu's hoverboard

36 rotors, 5.63 kW each
Total power: 203 kW = 272 hp
Payload: 1 person

In conclusion, ArcaBoard is nothing else but a badly designed hoverboard that requires an enormous power, 272 hp, to carry a man while 48.3 hp are enough for such a job, as demonstrated by C. A. Duru.
Source: A multicopter with 36 propellers that carries a man

Interesting find, +1. Clever to test it over water: No obstacles, and it hurts less if something goes wrong. Regarding the ARCA: I expect the lift to be less than what you calculate due to interference between the props.

I do not know how to take into account that interference between propellers. Anyway, a 5.7 kW EDF (see: "Schubeler DS-77 HST 120mm Carbon EDF Ducted Fan + DSM6745-700Kv Motor") delivers a measured 8.97 kgf max thrust. According to the formula I used a 5.7 kW, 120 mm propeller can lift 8.97 kg if the overall efficiency electrical + mechanical is 88% ((1.2 kg/m^3 x (5.7 kW x 88%)^2 x pi x (120 mm)^2/2)^(1/3) = 8.97 kgf). Likely the interference between EDFs diminishes the efficiency below 88%.

@PeterKämpf: The formula used is for calculating thrust produced by EDF - impellers, not propellers - so the interference is probably already accounted for by the formula.


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Is "lower pressurization" the correct term for the 787 Dreamliner?
In this blog, the FA refers to the 787 Dreamliner as having "lower pressurization":
http://theflightattendantlife.com/the-boeing-787-8-a-flight-attendant-takes-you-on-an-inside-tour-of-the-dreamliner/
The Dreamliner has higher air-pressure in the cabin due to its fuselage construction -- but this higher air-pressure makes the perceived 'altitude' lower.
Is the correct (air transport) term for this "lower pressurization" or "higher pressurization" or is there a better third alternative??

Neither. Both ways it is confusing.

I take it as an error - probably confusion between "higher cabin pressure" and "lower cabin pressure altitude". Maybe it was originally written as "lower cabin pressurization altitude" and someone decided it was too wordy or sounded too technical and simply dropped words without understanding how the change affected meaning.

I'd say that it depends on the official unit used to measure cabin "pressurization" (meter or pascal), if there is one. But this seems undefined.

The terminology is confusing. When you say lower or higher, it is better to clarify the reference.
Boeing 787 has a higher cabin pressure compared to the legacy airliners.
The best way to say it is that it is pressurized to (or has) a lower cabin altitude compared to other airliners.


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How is Calculated TakeOff Time (CTOT) calculated?
This answer mentions:

How these CTOT times are calculated is very complex, and deserves
  another question if you are interested.

I'm interested. How does it work? What is the level of complexity involved? What factors must be considered? If possible, what is the complexity class (P, NP, NP-complete, etc) of this problem?

Sounds like constraint scheduling, very likely to delve deeply into np-complete territory to get optimal things.

@ratchetfreak Yeah, I figured that with this many moving parts it would quickly get into np-complete.

I do not know the exact algorithm used for calculating CTOT's, but I can give you some background information about what information is used, and how pilots and air traffic controllers use CTOT's.
A CTOT, or calculated takeoff time, is a tool used within European air traffic flow and capacity management (ATFCM) to regulate the flow of air traffic through individual enroute sectors or airports.
The overall purpose of ATFCM is to prevent overload and ensure that capacity is utilised to the maximum extent possible and subsequently to ensure a fair distribution of delay.
Every area control sector and airport in Europe has a declared capacity, expressed as a maximum number of flight per hour. For enroute sectors, the capacity is determined by factors such as size of the sector, complexity of the route structure and military areas. For airports, the capacity depends on the runway and aerodrome layout, available approach types, weather etc.
For every IFR flight taking place within Europe, at least 3 hours before departure, a flightplan has to be submitted to the Integrated Initial Flight Plan Processing System  (IFPS), which is based in Brussels and Paris. The system will check that the flightplan is in correct format, and that it complies with any restrictions that may have been published, affecting the flight (for example, a certain route may only be available in a certain range of levels due to military activity).
The IFPS will process all these flightplans, and based on this information calculate the number of flights in any given sector or any given airport in Europe. If the number of planned flights (demand) exceeds the declared capacity, IFPS will start issuing CTOT's. A CTOT is a precise departure time, at which the affected flight must depart in order to ensure that, when the flight reaches the overloaded sector or airport, the demand does not exceed the capacity. This has the advantage that flights will spend their inevitable delay on the ground at their departure airport, instead of in airborne holdings. The IFPS will distribute delay fairly between flights. If an airport can handle 44 flights per hour, and there are 46 flights planned during some hour, the system will issue small delays to many flights, instead of just giving large delays to two flights. In case CTOT's are issued to flights as a result of an overload during a single one hour period, CTOT's will also be issued to flights planned in one hour before and one hour after that time, to avoid simply pushing the problem forward one hour.
In order to calculate these CTOT's, the IFPS must have access to up to date information about all flights. When submitting a flightplan, the aircraft operator includes an estimated off block time (EOBT), which is the time at which the aircraft is planned to leave the gate at its departure airport. If a flight is ready to leave its gate more than 15 minutes before the original EOBT, or is delayed more than 15 minutes, the airline must submit a new EOBT to the system, since the new departure time might result in an overload somewhere along the route. This may or may not result in the issuance of a CTOT time. Similarily, if a flight has been given a CTOT, but is unable to depart within the CTOT tolerance, which is -5/+10 minutes, the airline or air traffic control must submit a delay message to the IFPS, and a new CTOT will be issued. Obviously, IFPS must also be informed of any cancelled flights.
IFPS must also be provided with information about average taxi time at airports. The taxi time is the time it takes from the aircraft leaves the gate (EOBT) until it is ready for takeoff at the runway. During winter time especially, where de-icing operations at airports may reuslt in a very long taxi time, updating the taxi time in IFPS is very important. This is done by ATC. IFPS must know the taxi time because for each flight is only knows the EOBT time. Obviously, the system should not issue CTOT's that are earlier than EOBT+taxi time, since the aircraft would not be ready for takeoff at that time.
Air traffic controllers must depart any flights that have received a CTOT within a tolerance windon of -5/+10 minutes. If a flight is ready to depart before its CTOT, ATC must either have the aircraft wait on its gate, or some other place at the airport. At many airports, aircraft cannot be allowed to wait at the gate, because other arriving aircraft need to use the gate. It is then up to ATC to figure out what to do with the CTOT restricted flights until they can depart.
If a flight misses its CTOT, ATC is not allowed to issue a takeoff clearance. A new CTOT must be requested through IFPS, and the flight will potentially be delayed.
When a flight has received a CTOT, it is not set in stone. IFPS may send a slot revision message (SRM) if, for some reason or the other, the system has calculated a new CTOT. This can be both a better (earlier) or worse (later) CTOT. IFPS may also send a slot cancellation message (SLC), in which case the flight may depart without delay. Flights that are ready to depart within a short period of time, but are subject to a CTOT delay, can ask ATC to send a ready message, indicating to IFPS that if an earlier CTOT becomes available, the flight will be ready for it. In addition, an airline can request to swap CTOT's between two of their own flights, if the airline finds it more important than one flight arrives on time compared to another.
Finally, some flights are exempted from ATFCM restrictions, meaning that no CTOT delays will be issued. Examples of such flights are ambulance, fire fighting and search and rescue flights, and flights carrying heads of state. Flights departing outside of Europe will not receive CTOT's either, because no global ATFM system exists (yet) - but of course IFPS still has information about these flights, and they will be included in the calculations.
I hope the above gives you a better understanding of how air traffic flow and capacity management works in Europe. See this related question for information about ATFM in the USA.

Is it possible for the ATFCM to issue 2 overlapped slots (CTOT -5/+10) for 2 flights?

@FarahAmawi Yes.

In the United States a similar program is used. The initial program is a Call For Release in which the controller at the airport will call a Traffic Management Specialist at the overlying center for a release time for a specific flight; the release is valid for -2/+1 minutes around the specified time. More severe is the Expect Departure Clearance Time program, where all flights to an airport will have pre-computed EDCTs; they must be airborne ±5 minutes from the EDCT.


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Why does the F-35 Lightning II have only 1 crew when the F/A-18 Hornet had 2?
Many successful fighter aircraft, eg F/A-18 Hornet, have 2 crew (pilot and navigator), which (from my albeit limited research) gave great benefit in complex situations especially involving ground defences.
The F-35 Lightning II is supposed to be multi-purpose, but has only 1 crew - why?

Do the advanced electronics compensate for the lack of a navigator?
Would the design (aerodynamics/frame) be compromised by adding another seat?
Something else?

I found that duplicate, but I don't think it is s dupe, because this question is specific to the JSF

@Bohemian: You should check the answer to the linked question by KeithS. Notice by the development of the F-15 through F-18 that designers and US top brass were already considering single-seaters to be sufficient; - the answer extensively lists what necessitated the second crew member and which advances made them redundant. Hornet was a notch above the line, F-35 is firmly in 1-seater territory.

F/A-18A and F/A-18C are single-seat airplanes.  F/A-18B and F/A-18D are two-holers, primarily used for training.  F/A-18E and F/A-18F are significantly different airplanes, with the E being a one-holer and the F being a two-holer.  F/A-18G is an electronic warfare version of the F/A-18F.  So it is not correct that "F/A-18" is a two-seater.

Why does the F-35 Lightning II have only 1 crew when the F/A-18 Hornet had 2?

According to program officials involved in the development of the F-35, the F-35 is a single seat aircraft because:

New advances in avionics and sensor fusion technology, which can
  allow  a single pilot to perform missions that previously required two
  crew members.

Given the fact that the F-35 is build around the concept of
Network Centric Warfare there is less cockpit workload for most strike missions. Consequently, the need for a co-pilot is a lot less.
The single-seat configuration also maximizes commonality between the
three variants of the F-35 thereby keeping development costs low.
The survivability of the F-35 includes limiting the probability of   detection. Ergo, a two-seater version can very easily increase the RCS of the F-35 thereby compromising its stealth capabilities

West 2014: Joint Strike Fighter: What Do the Pilots Who Are Flying It Today Have to Say?
That said, one major drawback that remains on the one seat design is the demand on one pilot to assimilate and decide on all the information available.


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Why are C17's horizontal stabilizer mounted so high?
The C17 has its horizontal stabilizer mounted at the top of the vertical stabilizer, way above the plane of the wing

but most aircraft, including other large aircraft made by the same manufacturer (Boeing), mount their horizontal stabilizer on the tail of the fuselage, very close to the plane of the wing, for example the 767:

Placing the horizontal stabilizer so high requires the vertical stabilizer to be much stronger, adding weight and stress, so there must be a benefit/reason for designing the C17 like this.
What benefits/reasons are there for aircraft to have the horizontal stabilizer mounted this way?

Probable duplicate of https://aviation.stackexchange.com/questions/54424/why-do-the-c-141-and-c-5-have-t-tails/54509#54509

@ThatCoolCoder Yup. It's a dupe. Thx.


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Use of a solenoid valve over a selector valve
When designing a low-wing light aircraft with two integral tanks at the wings and a collector tank in between, what type of valve should be used to allow the pilot to select the integral tank which will feed the collector tank in a reliable way? A selector valve or two solenoid valves?
Fuel will be fed from the integral tanks to the collector tank via jet pumps with no moving parts, not gravity.

Hi Enes, welcome to this site. Are you asking about purely mechanical selector valve vs. electric solenoid valves?

Yes, that is exactly what I am asking for

This question, the way it's written, is highly open to opinionated responses. The answer to this or question is "yes". It depends so much on the design. If it's an ultralight/LSA and this saves 500g which can then be moved into structure, then it's quite possibly critical choice. If it's a humdrum standard airplane where weight and complexity decisions don't arise because of regulatory limits, then the answer might be different. @EnesSenel, with this in light, could you reword this question to make it so there's a clear "right" and "wrong" answer?

There's at least one strong reason to prefer manually operated valves over solenoid valves: they still work during a complete electrical failure.
Aircraft engines, due to magneto ignition, will continue to operate even with (for instance) an exploded battery, melted main ground cable, or other failure that kills the entire electrical system and can't be repaired in flight.  You'll have no landing lights, instrument lights, clearance lights, radios, or electical instruments -- but at least you'll have power, basic vacuum or airstream powered instruments (airspeed, altimeter, turn and bank, assuming these analog gauges are installed) and presumably if you have retract gear you have a manual method to lower it; therefore you can continue to fly and land the airplane (at least in daylight VFR conditions) -- but with solenoid valves, you may be limited to the fuel in the collector tank or, with a better design, you'll be on "both" willy-nilly or feeding from one wing without the ability to switch (leading to trim problems as fuel is depleted).
Bottom line, a fuel valve that works any time you can keep the engine run is better than one that quits if the master switch gets flipped.

Inability to shut off fuel flow in an emergency involving loss of electrics is frowned upon, to say the least. To create a setup that leaves fuel lines open with no electricity will keep fuel flowing and engine running, but you'd be essentially landing an incendiary bomb. Emergency landing procedures commonly list "fuel valve off/closed", I'm guessing this may very well be a regulatory issue as well: you'd need a fail safe (mechanical) fuel shutoff anyway.

I agree, though the shutoff need not be capable of selecting either tank or both in a dual tank setup -- but having the engine shut down for lack of fuel if the master breaker flips is nearly as bad (an avoidable forced landing is something I've understood to be undesirable).

Solenoids can have multiple stable positions. It's quite the big assumption that the designer is considering defaulting valves to "normally closed".

@KennSebesta Sure, but you're unable to change the state.  You may be stuck with a full right tank and dry left, and have to do a forced landing with a heavy wing because you can't switch feeds.

This is easily designed out by having valves which fail "normally open". Then an electrical system failure simply results in drawing from both wings. In every plane I've ever flown with a "both" selector, I've never once used one tank exclusively.

@ZeissIkon In principle I agree with the concern that electrically actuated valves can get stuck (although reasonable arguments could be made as to whether this is a "failure"), although the same thing can happen with a mechanical valve. As an aside, there's a lot of fear in aviation of complete electrical system failures, but now that people are paying attention to this aspect of design it looks like those problems are disappearing into the past.

@KennSebesta Well, like any other "whole aircraft" failure, it's not common, but if it happens, it can be really bad.  I did have a battery explode once, in a car, but it was on a start attempt, which would usually be a ground event in an aircraft, so would prevent flight rather than leave you stranded at altitude with most of the panel dead and no lights.  I agree, fail open with a separate mechanical shutoff would be the sensible way to design...

You would use a solenoid or other electrically operated valve, like a torque motor operated one, only if you must use it remotely and it isn't practical to operate it mechanically.
In doing so you also have to provide redundancy in the form of a manual override (such as a cable you can operate that forces the valve open following an electrical failure in the valve or upstream of it, or a sticking solenoid), or dual valves in parallel (or whatever else you can design to eliminate safety-critical single-points-of-failure).
Solenoids like to stick and torque motors like to burn out, and even if the valve is spring loaded to be normally open so it is fail-passive in theory, I wouldn't trust my life to it without some form of backup.
In a light aircraft design where the components can be made to be easily in reach of the pilot, a manually operated selector valve, or manually operated shutoff valves at the wing roots is the way to go. There is simply no point in designing an electrically operated system when a simple and robust manual alternative is easily done.
My preferred design would be a left-right-both selector right at the collector tank entrance, a separate emergency shutoff valve at the firewall operated by its own remote mechanical control that sticks out and gets in the way of things when at OFF, and ideally, manually operated emergency shutoff valves at the tank outlets in each wing so that fuel is contained within the wing if it gets broken off in a crash (that may be overkill - I've never seen shutoff valves right at fuel tanks in light aircraft, but I'd do it if it was my own design).


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Why aren't larger airliners such as the A380 more efficient than smaller ones?
A recent Washington Post article, titled "Five myths about air travel" noted:

The difficulty is that economies of scale don’t always work for airlines, because planes generally don’t increase in per-passenger efficiency as they grow larger. In fact, many of the most efficient planes of today are the smallest ones. At a transatlantic distance, a 525-seat Airbus A380 has an efficiency of 74 miles per gallon (mpg) per passenger, while the brand-new 168-seat Boeing 737 MAX 8 reaches 110 mpg per passenger.

This seems counter-intuitive. All other things being equal (and transatlantic distances would equalize many of them) -- shouldn't the larger plane should be more efficient?
This whole article notes the "myths about air travel" and I have to admit that I'd always believed that larger planes were more efficient per passenger mile.
Perhaps there is something unique to the the A380 and 737-Max 8 that makes the comparison incorrect? Is it related to aircraft utilization rate or percentage of seats filled -- although these wouldn't be a factor in the mpg per passenger metric?
Note: I am not interested in crew or ground staffing efficiencies realized by a mega plane. Just fuel.

Shouldn't the unit be passenger miles per gallon, rather than miles per gallon per passenger? If they can't get units right, that makes me dubious about the other claims.

Another difference is 12 years of innovation (between the first flight of the A380 and the 737 MAX 8) during which much of that time oil was above $100/barrel and low fuel usage was a primary design consideration. A lot of engineering effort went into reducing fuel consumption for the 737 MAX and the A320Neo. The A380's fuel consumption isn't that out of line with similar aircraft of the same vintage either.

Smaller aircraft need more crew members (per passenger), more ground crew, probably more maintenance. They increase traffic load, ATC load and delays which is money too. I don't know about the impact of this, but it may be taken into account for a fair comparison.

@Sanchises Technically, aren't "miles per gallon per passenger" what you would get if you took a number of miles per gallon and divided it by a number of passengers? If your vehicle gets 10 miles per gallon and has 10 passengers, that's 1 mile per gallon per passenger (a useless measurement) but 100 passenger miles per gallon. All that said, the term "miles per gallon per passenger" did sound right to me at first glance.

@Tanner You're right. I was thinking in l/100km instead of mpg, a typical measure of vehicle efficiency on this side of the pond. Deleted comment.

I think what's missing from the analysis is range. If you need to fly a 9000 nm route you have to carry 9000 nm worth of fuel. And you'll have to have a bigger plane just to carry that fuel - regardless of passenger load. That will inherently make for inefficiency. You don't see airlines using an A380 for short hops. If they need to carry 500 passengers from Pittsburg to Cleveland they'll fly 5 regional jets rather than one super jumbo

@TomMcW The question specifies transatlantic routes so (JFK,BOS) to (LHR, FRA, CDG), etc. so about 3500 - 3700 miles.

@RoboKaren It's a bit tough to find 737's on transatlantic flights. There are only a handful on unusual routes and they usually have to limit the number of passengers to get the necessary range. Until recently they just haven't been capable of the range. Although there are numerous other considerations, for fuel efficiency airlines will generally use the smallest plane capable of the flight range. They don't make bigger planes for the extra seats, it's for the extra range.

I remember a question or answer here on aviation which stated that making planes bigger (within restrictions of what physically is possible) is more efficient in general, because I requires an decreasing amount wing-surfance per passenger. If someone found this, please post it or tell me that I'm wrong :)

The comparison is seriously flawed.
While over the entire duration of the flight the larger aircraft is more efficient per passenger/mile, this is not the case for the flight phases that cause the bulk of the fuel consumption for smaller airliners but only a fraction for larger airliners: takeoff/climb and descent/landing.
A 747 or A380 flying a one hour leg, which is typical for many smaller airliners, would use more fuel per passenger over that leg than does say a 737 or A320, despite being more fuel efficient at cruise altitude (if it can even reach its optimal cruise altitude over that distance).
It is also far less flexible, and requires a higher passenger density than does the smaller airliner, a passenger density that's simply not there on most routes unless you greatly reduce the frequency of flights (in which case you'd best hope all your competitors do the same as passengers are less likely to be happy to wait for your one flight a day at noon when they can get to their destination at 8AM taking another airline's flight).

The question specifies transatlantic flights, which are usually four or more hours in duration.

@RoboKaren It doesn't matter much. Not over those distances. And the economic argument still stands. There's a reason airlines go with smaller aircraft and it's because there's no point flying an A380 on a route that only generates enough traffic for a 787, it'd fly with half its seats empty, making it far less efficient.

update I could read the article just now. It does not consider how many passengers need to be transported. For one A380 you need three B737MAX to fly transatlantic, with 6 pilots etc etc. What they are doing is comparing aircraft per aircraft, and yes a huge plane like the A380 needs a lot more thrust and needs to burn more fuel. But they should be comparing one A380 with three 737s if considering moving 560 people to the other side of the pond.
Range is a variable here as well. How to compare air travel economy for ranges that the A380 are designed for, let's say Paris - San Fransisco. Do the 3 737s make a fuel stop in New York?
The bit on numbers of engines of comparable types is still valid, but mostly for aircraft of comparable size:
The best practical case study is the one performed by Airbus: the same aircraft, powered by either two engines (A330) or four engines (A340). The A330 is still relevant and being updated with NEO engines, the A340 is out of production - not because it was a bad aircraft but because the restrictions on ETOPS have been relaxed. Number of engines is a huge factor, fewer engines means higher efficiency. Each additional engine adds drag from the engine pods, one engine weighs less than two half size engines etc.
When the A380 was designed there was talk of a stretched version, the A380-900. A stretch is easy if the only thing required is to plug in an extra bit of fuselage, but gets very hard if a complete re-design of the wing needs to be considered. It looks like the current A380-800 has the larger wing of the -900 already, in other words: that wing design was optimised for the higher gross weight of the -900. That would be a very economical aircraft - if a big enough market could be found for it.
The B-777 is currently the largest passenger jet with 2 engines. The air travel myth article should consider total traffic volume, and compare the cost per passenger based on that. Economy of the A380 is actually pretty good, but sales of the aircraft seem to be hampered by lack of high traffic volume routes.

It's still all about range. The A340 was discontinued because of reduced demand for longer range flights. After a certain distance the A340 made up for the extra weight and outperformed the A330.

Air Asia could not make the Kl -Lomdon flight work with A340s due to fuel economy, they were holding out for A350 to re-open routes to Europe.

The A340 was designed to counteract the very restrictive ETOPS rules in force at the time, which prevented aircraft with two engines from flying many overwater routes that they could otherwise fly (based solely on their range). Fortunately (or unfortunately for the A340) ETOPS extensions (allowing twinjets to fly longer distances away from airports) were approved faster than anticipated, negating the A340's advantage over twinjets.

Yes indeed, it made sense at the time.

Which is also a prime reason for the untimely (or timely if you're an aircraft mechanic) demise of the MD-11. It was given 3 engines to not need ETOPS classification.

The primary reason is the gatebox limit at airports. For an aircraft the size of an A380, to be as efficient as a smaller aircraft, it requires a much larger wingspan to avoid the induced drag penalty associated with smaller aspect ratio wings. The gatebox limit at airports prevents this increased wingspan resulting in much lower efficiency specifically for the A380. This has been well documented and I would recommend looking at the configuration aerodynamics notes made available online by the late Prof. Mason of Virginia Tech.

Do you have a link supporting your assertions?  googling "gatebox limit" didn't turn up anything relevant.

@OrganicMarble I guess they are referring to the ICAO Aerodrome Reference Code Element 2 or the Airplane Design Group. Both limit wingspan to 80 m in the highest category (A380 has 79.75 m wingspan). Nonetheless, this answer would benefit from some references.

The size of gate needed to plane/deplane pax has nothing to do with the passenger MPG the plane gets in the air.


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Do planes return to service after non-fiery crashes?
A Turkish Airlines flight recently ran off the runway and ended up on its belly in a field. The landing gear were damaged, but the plane didn't burn and the fuselage remained intact.
Are planes involved in this sort of accident repaired and returned to service, or are the planes usually scrapped?

Boeing has a dedicated team for this. Of course they don't refer to "crashed", it's called the Airplane on Ground (AOG) team.

If the aircraft can be economically repaired (there is no major structural damage, and we're not talking about an aircraft already scheduled to be retired) then it's likely the aircraft will be repaired and returned to service. When you spend several million on an aircraft you aren't going to throw it away for a few tens or hundred thousand dollars worth of damage, particularly as it's probably insured.
If the aircraft is structurally compromised to the point where repairs would not be economical (or if we're talking about an older aircraft which has given many years of service and is due for retirement) it would probably be written off and scrapped for any salvageable parts (either repairing it to a condition where it could be ferried to a "boneyard" or dismantled and cut up at the accident site - whichever option is less expensive for the airline).

As one particularly famous datapoint, the Gimli Glider had an off-airport landing with a nose gear collapse in 1983. The aircraft was repaired, returned to service, and flew until 2008 (a little under 25 years of additional service after its incident).

A couple of years ago I was waiting at the gate for a transatlantic flight when the registration of the aircraft I was about to board was suddenly familiar to me (C-GITS), but I couldn't remember what I knew it from. Quick googling revealed it was the Azores glider which is still in service today. This time it safely reached the other side of the pond.

One of the more remarkable examples is JAL Flight 2. In 1968, a DC-8 landed in the bay 2.5 miles short of SFO due to pilot error. Nobody was injured, the aircraft was refurbished at the cost of many millions of dollars, and it returned to service for several decades (it reportedly went to salvage in 2001).

@DeltaLima, while trusting in the process, that had to be at least slightly disconcerting...

And needed to say, Gimli glider had the fuselage quite damaged in the nose section, but it was local enough.

It's also worth noting that the converse holds: sometimes what seems like relatively minor damage will result in a plane being scrapped. I believe the Southwest plane that lost its nose gear at LGA SWA345 / NTSB File DCA13FA131 was scrapped as Southwest felt the needed repairs were uneconomical on a 15-year-old airframe.

To be pedantic, the Gimli Glider didn't make an off-airport landing.

@jamesqf It made a "former airport" landing - I believe the drag strip had been removed from the chart at the time it landed there :)

Yes, some even after a fairly fiery crash. Here are some photos and a video illustrating some very interesting cases (ALCI Lidia 2012-13 crash and recovery and ALCI Mia accident in 2009).
Here they even say that the airframe was totaled ten years prior to this accident and repaired a year after that.

Edit: Factual corrections about the video (thank you @DeltaLima).

The video is not from the Lydia crash recovery, but from the Mia crash recovery. Very interesting to see though.

@DeltaLima Thanks, I'll edit the answer.

Holy cow! They actually rebuilt that plane in the field in Alaska? That must be an expensive plane if recovery efforts in those conditions was economical.

@FreeMan I've seen suggestions that they were going to have to move the airplane regardless of whether it flew on its own power or was cut up and trucked/flown out.   With that constraint it was probably worthwhile to get it airworthy.  Also note that wasn't Alaska, it was Antarctica.

I don't know about large aircraft, but for smaller aircraft it is usually dependent on whether a structural member was damaged, especially a bulkhead. If a structural member is bent or crushed, then usually a small plane will be unsalvageable, but if structural members are intact and undamaged, then usually the aircraft can be repaired.

This also depends on WHICH structural member: As you've noted a crushed former/bulkhead might do a plane in, but there are some light planes flying around that have had wing transplants (something happened to bend the spar, so unbolt the wing, bolt a new/salvaged one on, and keep flying).

@voretaq7 I used the word "usually". Every crash is different.

I'm not going to argue english-language semantics with you - I made the comment because your answer seemed incomplete and likely to give an incorrect impression. Feel free to incorporate my clarification/expansion into your answer, or not, as you wish.


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Why do most helicopter pilots sit on the right?
A follow up on this question, one answer mentioned that

Please note that on many helicopters, captains sit on the right side.

Why is that?


text:
How often are passenger oxygen masks deployed?
In commercial passenger aviation, how frequent (or infrequent) is the case that oxygen masks deploy?
Please include links to statistics.

So infrequently that any statistics, if they exist, will be meaningless.

Not statistically significant, almost no matter how you slice it. Flying Hours/Deployments, Flights/Deployments, Aircraft/Deployments...

@RalphJ: Opinion, because, in my opinion, there is no mandatory notification for such event.

@RalphJ Are you expecting an answer which is not opinion based?  Any answer, since there will be no meaningful measures, must be opinion based.  I'll kick it off.  In my opinion, once every 12.7 million person miles.

@mins Couldn't one work out an upper bound by looking at, e.g., cabin pressure loss? http://www.avherald.com/h?search_term=Cabin+pressure&opt=256&dosearch=1&search.x=-862&search.y=-106

@Keepthesemind: In that case it would be appropriate to change the question and ask directly for pressure loss events. It would be less ambiguous (e.g. the crew can deploy masks by accident by pressing a button in the flight deck, regardless of the pressure.)

I'd be willing to bet serious money that there are far more deployments from hard landings and accidental deployments than from loss of pressurization. I'm embarrassed to admit it, but I made a hard landing in a 727 back in the 1980s that dropped a few masks in the back, and once while flying right-seat in a Citation, the left-seat guy accidentally deployed the masks. Neither I nor, to the best of my knowledge, any of my flying acquaintances ever experienced a loss of pressurization resulting in a deployment.

This is a good question. In my regular training I have to provide an answer to the underlying question (how frequent are aircraft depressurizations?) and in extensive research I have not been able to come up with a concrete answer, either current or historical.

helpful post http://aviation.stackexchange.com/questions/34177/how-likely-could-there-be-a-loss-of-cabin-pressure

Very, very rare, probably in the range of sinlge digits per 100,000 flights (for major first-world airlines, perhaps higher elsewhere).
I'll try to get you a more numeric answer tomorrow.
Posting an answer now so that I can edit it, evem if the question gets closed between now and then.
"Opinion based", horsefeathers!

try to help you: militar: https://www.ncbi.nlm.nih.gov/pubmed/15945394

australia: https://www.atsb.gov.au/publications/2006/b20060142/ and https://flightsafety.org/wp-content/uploads/2016/12/asw_oct06_p33-41.pdf (accidents per year)

canada: depressurization: http://www.bst-tsb.gc.ca/eng/stats/aviation/2012/ss12.asp#sec4

finaly: researching in the NTSB records, 01/01/2011 to 31/12/2016, there were 12 itens for "oxygen masks".


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Where to find information on ILS categories at European airports?
I have a list of airports in Europe and I need to find up-to-date information about ILS categories (I, II, IIIa, IIIb or IIIc) at these airports. Does anyone know where I could find such data efficiently?

There is no single repository, if this is what you are after. Like for the US, each country has its AIS which publishes an AIP. The information is within individual airport information in the AIP. E.g. the AIP for France, and an ILS Cat I-II-III approach for De Gaulle 08R.

Try here: http://www.aviationwap.com/SearchForm.php

The information you are after is, unfortunately, rather distributed.
Eurocontrol maintains a useful list of websites ordered by state, found here:
European AIS
Or by registering for an EAD Basic account you can access the European AIS Database in a more centralised and user friendly way, found here:
European AIS Database


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What is the freezing point of 100LL Avgas?
Does anyone knows the exact freezing point of Avgas (100LL) ?
I found that Jet A-1 freezes at −47 °C / −53 °F (wikipedia), but didn't manage to find any info on Avgas.

Are you asking for some particular purpose (like you're designing a system where you need to keep avgas from freezing), or out of curiosity?

@TannerSwett Just out of curiosity ^^

Avgas, which is gasoline, must sustain -58°C, this table from ACRP Overview of Airport Fueling Operations

ExxonMobil China says their Avgas 100LL will not freeze above -58°C.

The 'typical' freezing temperature is not given, but I would expect we'd be more interested in the maximum freezing temperature.

@ymb1 Nice edit. How did you do that?!


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How are north and east compents of the wind calculated, given a wind speed and direction?
I have a wind speed and wind direction of an aircraft. I need to find wind component along the true North axis and East axis.
Can anyone explain me how to do that?
Say wind speed is the hypotenuse in the Right angled triangle and angle is wind direction. How to find wind speed along north axis and east axis?

Take a look at the trigonometric functions sine and cosine.

SOHCAHTOA

Possible duplicate of How does a crosswind affect an aircraft's speed relative to the ground?

Simply use trigonometric functions:
$V_{north} = -V \cos \theta $
$V_{east} = -V \sin \theta $
$V$ is wind speed
$\theta$ is wind direction clockwise relative to the true north (e.g. east is 90°). Note that wind direction is typically given as the direction from which the wind is blowing. If you have a wind direction that in given into the direction of air flow, then you should remove the minus signs from the formula's above.

@DeltaLima Does this hold true for all values of θ (0 - 360) ?

@geetha yes it does. If you implement this in a computer don't forget that you have to convert the angle from degree to radians before using the trigonometric functions


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Can you help me to identify this helicopter?
I came across this helicopter and am wondering what the oblong things on either side of the helicopter are, parallel to the front-back axis, offset from the main body.

Can you post a better photo? Maybe some zoom & other image enhancements?

@curious_cat How's that?

Much better, thanks!

@andrewmh20 Can you tell when and where u came across that heli?

@aeroalias Israeli skies, and I see from the link below that Israel had some CH-53D's.

@andrewmh20 I had a doubt it was not a Pave Low as they've been retired. Your comment confirmed it. I've changed my answer accordingly.

The helicopter is a Sikorsky CH-53 (probably an 'E' model).
According to this site, the two sponsons holding the fuel tanks are also the place where the landing gear retract.
EDIT: Probably not an 'E' model as the 'E' model has a seven-blade rotor, while the one in this picture doesn't.
I'd say this is a 'D' model or an MH-53 'J' model.

the drawing from wikipedia matches the shape quite well: https://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/CH-53D_Sea_Stallion_Drawing.svg/2000px-CH-53D_Sea_Stallion_Drawing.svg.png

@Federico, edited, because of model mismatch...

@MarcoSanfilippo CH-53D makes sense with location --see comment on question.

The helicopter is a S-65C-3 Yas'ur (Albatross). It is a CH-53D version similar to USAF HH-53C Pave Low. Here is a photo of the same.

The identification is based on the following:

Six bladed main rotor
The sponsons which hold the fuel
The only variant of CH-53D with in-flight refueling probe in service is the Israeli version. All others (HH-53 and MH-53) have been retired. CH-53E has refueling probe, but it has 7 blades.

Israel ordered 36 of these,of which around 23 are still in service.


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How do pilots see at night if airplanes don't have front lights?
How can pilots identify threats ahead of them if they don't have front lights like cars?

They do have lights to see at night, but it's called radar.

@mins Radar isn't required for all civil IFR flights, but many have it. Also, in general, any commercial passenger or cargo flight on a big enough plane (at least 20 or 30 passenger seats or above some weight, and with turbine engines) is going to be required to have a TCAS radar to detect other aircraft and avoid collisions.

Even very small planes have a very bright "landing light". It is the most expensive light bulb you can imagine and it breaks all the time and is a complete pain in the butt. Imagine buying a $150 light bulb, putting it in a shop light and then banging it on the ground until the filament breaks. Then buy a new one. Keep repeating. Thats what its like owning an airplane.

@TylerDurden - Do you think LEDs will get good enough in the next few years to replace this incandescent bulb anytime soon? We're really close to LED main headlights on cars and we already have LED strobes on planes (same basic assembly as camera flashes). I realize this is a still higher-lumen application, and rather specialized, but with "aviation markup" being a normal thing, I'd think owners would be clamoring to pay $500 or so for a landing light that promises a 10-year service life.

@KeithS LED lights are already good enough. The problem is that it is "non-standard" equipment so it is a regulations violation to use them.

Short answer: they do have front lights!

Longer answer: There's lots of stuff for a ground vehicle (such as a car) to hit, because there's lots of stuff close to the ground.  There's not much in the air for an aircraft to hit, besides other aircraft.  Lights on an aircraft in flight are to help the airplane be seen, not to help the airplane see.
On landing, forward-pointed lights are nice to have, but since runways are illuminated, it's possible to land safely without them.  Airliners in particular will probably never be landing on a darkened runway and need to worry about hitting invisible terrain or obstacles ahead, because they're always either above the obstacles or they're on a known safe descent path.

The landing lights can be helpful for seeing, say, the stray dog on the runway, though. :) In general, though, yeah, they're more useful for being seen than anything else. For that matter, headlights are useful for that purpose on cars, as well.

At the speeds airplanes travel, headlights wouldn't be much use to illuminate stuff in front of you, anyway.  Most people overdrive their car headlights at night, and light planes typically travel at least twice as fast.

@reirab - I used to tell students in our simulator that I had spotted a dog on the runway when I wanted them to execute a go-around.  One time a student responded "Don't worry, we'll win." and landed.

@jamesqf That's not really true for GA aircraft. At touchdown (when the landing light is useful,) the Cherokee I fly is slower than I normally drive on the highway. At cruise, it's significantly faster, but there (thankfully) aren't many obstacles at cruise altitude.

@SteveV. That's great. I must admit that I have actually landed on a runway that had dogs on it, though (a real runway, not a sim.) The dogs entered the runway after I touched down, but I exited the runway before I got anywhere close to them.

@reirab: Yes, that's what I meant, that headlights wouldn't be much use for seeing obstacles at altitude.  (And are in fact often counterproductive, with reflection from any haze destroying your night vision.)  Landing of course is a different matter, or even doing a low pass beforehand to scare the deer/cows/whatever.

@SteveV. Any special requirement for aircraft to be certified for night flight? If yes, maybe you can add that to the answer :)

That's a good picture, but if you really want to show off some bright landing lights, you should use a more appropriate aircraft....  like, say a Pilatus (I swear, every time that I see one I think that it's a 747).

Pretty much anything you want to avoid in the air is (or should be) lit. Other airplanes, radio towers, that sort of thing. If it's not lit, then either you couldn't have done much to avoid it anyway (bird), or you should be able to anticipate it (terrain).
Airplanes do have landing lights, which are quite bright and point forward. In the US, commercial flights are required to have them on under 10,000 feet. They are used to illuminate the runway for takeoff and landing. Planes should not be close enough to illuminate anything with their landing lights until landing though, and they are required to be on to allow aircraft to see each other more easily.
Taxi lights are similar but generally less bright, and help the pilots see where they are going when taxiing on the ground without airport lighting.
This diagram of lights on the MD-11 shows the main areas illuminated by the taxi and landing lights. The other lights help to make the aircraft more visible as well. Runway turnoff lights on airliners also help to light up areas to the side of the aircraft after landing to more easily see upcoming runway turnoffs.
Source

Military aircraft and missiles could be added to the list in the first paragraph.

Great Diagram..

They don't really need to.
Hills and other man-made static obstacles are mapped and lit and height limitations are published to which the pilot can follow to stay safe as long as he knows were he is. Landing strips will be lit up for them.
Other planes will be lit up with navigation lights (red and green) and a pulsing beacon to draw the eye. Also during the night pilots will be flying IFR, which means the Air traffic control will be keeping planes away from each other.
And they hope no birds will be flying in their path.
A light on the plane won't be enough to light for obstacle any way and provide a nuisance on the ground for the people trying to sleep.

Maybe pilots should fly IFR at night, but they don't have to: night VFR is legal in many countries.

To expand on what Pondlife said, you don't even need an instrument rating at all to fly at night in the U.S. In fact, flying at night is a requirement to get a private license in the first place and private pilots are encouraged to do so regularly to maintain currency. Also, landing lights are much more useful for seeing other aircraft (especially during the day) than nav and beacon lights (assuming you're looking at the aircraft head-on, of course.)

@Pondlife:  I've been on several moonlight flights where I could see even more clearly than during the day.  With lots of moonlight, no sunshine glare, and each roadway and tower clearly lit up, every detail was perfectly visible.  Some nights may be IFR appropriate, but night VFR can often be optimal.

@abelenky Yes, absolutely. I didn't literally mean that you should always file IFR at night, maybe I should have written that "pilots can but they don't have to".

Many other answers have covered lights for seeing on the ground, which are invaluable during taxiing, takeoff and landing.  They've also covered how most obstacles (like other planes) should be illuminated to be seen (according to Advisory Circular 70/7460-1L in the US).  I'm going to cover the other ways that an aircraft can "see" what's around it. IFR navigation and avoiding controlled flight into terrain are related topics.
The answer depends somewhat on whether we're talking about visual flight rules or instrument flight rules.  Visually spotting lit obstacles is far more important if you're not instrument-rated.   Of course, planes don't have to visually see obstacles to avoid them. Properly trained pilots can fly IFR in zero visibility, so they have plenty of tools to identify threats ahead that they can't see.
Charts
Aircraft can fly in low visibility situations where even lights won't do you much good.  This is done in large part using careful navigation and reference to charts.  Sectional charts are required to show peaks, towers, and other obstructions as shown in the legend below.  Approach plates and procedures may also list smaller obstacles particularly close to the runway.

You'll notice that the sectional below has the obstructions marked, as well as a big 07 in the right hand bottom.  That 07 means that the highest terrain or obstruction in the right hand box on the image below (basically everything east of Logan Intl Airport, outlined by straight lines with tick marks) is 700 feet, including a small margin and rounding, so if you're flying safely above that altitude you can avoid obstacles.

Synthetic Vision
A new emerging technology is synthetic vision, as mentioned in SierraSkyport's answer.  This system combines precision location information with very detailed terrain and obstacles databases to provide a view of the surrounding area.
This is sometimes combined an enhanced vision system with an near-IR camera to see in darkness and through fog and clouds, and this clear camera-based view of the surrounding area is often overlaid on the center of the synthetic vision display.  I've seen a near-IR camera in action and you can actually make out features like the edge of the runway and buildings, even in very low visibility.

(Image by Honeywell, provided with Permission)
TCAS and ADS-B
Your major obstacles when above the ground are other aircraft (I don't know what else you'd hit in the air, meteorites?) Any aircraft equipped with a mode-C or mode-S transponder is "visible" to a plane with TCAS, and the TCAS system will provide traffic alerts for nearby aircraft (and guidance for how to resolve the conflict if the intruder gets close enough and it's a TCAS II system).

ADS-B in also provides a traffic information for any nearby aircraft equipped with ADS-B out (which thanks to mandates is becoming very common). If you have ADS-B out as well as ADS-B in, you'll additionally receive radar information for all aircraft from ATC over a service called TIS-B. ADS-B is often supplied integrated into a mode-s transponder, but this is not always the case, so you could have ADS-B out and ADS-B in but not a TCAS-compatible transponder.
TAWS, GPWS, and Radar altimeter
For avoiding the ground these systems provide some information to avoid a controlled flight into terrain.  Terrain  awareness warning systems (TAWS) and ground proximity warning systems (GPWS) (the two are sometimes synonymous) are now required by the FAA on all large turbine and turbojets.  These systems warn if you're descending too fast, below the glideslope, or flying too close to terrain, based on a radar altimeter.  Even without this system, you may have a radar altimeter to monitor your height above terrain.
Radar
Weather Radar (or Stormscopes), XM radio, and ADS-B all provide excellent information about weather ahead that you can't see.  Most non-military aircraft do not have primary radar for seeing things other than weather. Since ATC provides their own primary radar information over TIS (ADS-B traffic information), there is not much need to have a primary radar on the aircraft.
ATC
In a worst case scenario ATC has tools like minimum safe altitude warning systems and short-term conflict alerts and may alert you if you're about to run into terrain or another aircraft.  As far as I understand, this isn't supposed to be your primary method of avoiding obstacles though, even in IFR conditions.
Night Vision Goggles
Of course, if you really need to see far ahead of you in dangerous terrain, you can use night vision goggles.  Your aircraft needs to meet certain requirements and you need training, however.

Excellent contribution! In your Charts section, you call out the 07 "in the right hand box". Is that "box" the portion of the chart enclosed by the arc of the KBOS Class B(?) air space and the V-139-268/WIMPY airways? Not a pilot here, but trying to learn stuff.

I made it a little clearer in the answer.  It's actually the portion of the chart outlined by the ticked horizontal and vertical latitude and longitude lines.  You can find out more on http://klaviation.com/maximum-elevation-figure-mef-sectional-chart/ , see also the drawing in this answer: http://aviation.stackexchange.com/questions/22052/what-steps-can-pilots-take-to-mitigate-the-risk-of-controlled-flight-into-terrai

Got it. That makes much more sense. Thanks!

For civil aviation, planes are lit to make them visible, not so much to illuminate obstacles.
Military aviation is (or was) another matter, in WWII operations were sometimes conducted at night with upwards of a thousand unlit planes. Moonlit nights were to be avoided where possible, because moonlight made it rather too easy for pilots to see other aircraft, and the full moon was called a "fighter's moon". However, especially hazardous operations were sometimes scheduled for the full moon (e.g. see "The Dam Busters", Paul Brickhill) despite the extra risk to the pilots.
Maybe someone can ... illuminate us, as to whether this is still a consideration.

Military combat missions are (or at least were in the 80s) flown without navigation lights. Sure, everyone's got radar & infrared, but no need to make it any easier on the enemy than necessary.

For landing and take off, lights are essential.  It's possible to do both with just runway lights.
In the air, VFR pilots can still see ground lights and objects in the sky that are lit (other planes).  This allows them to see the horizon and keep the plane properly oriented.  Navigation is the same as it is during the daytime, with the only exception being that pilotage (flying by reference to objects on the ground) is limited.
During a full moon or nearly full moon and cloudless sky, flying at night is only marginally more difficult than flying during the day.
On a moonless night, or overcast, things are much more difficult.  Flying in an area where the horizon can't be discerned is particularly dangerous and should be avoided.  For those situations, flying IFR is the only safe alternative.

Headlights on a plane are pretty worthless (other than near the ground).
Just as your car headlights don't help you see a mile down the road, headlights on a plane aren't useful for illuminating objects beyond a few hundred yards.
It is more important that planes, towers, and other obstacles have lights on them to be seen, not to see.
That is why planes have red and green lights (as well as a beacon on top, and a white light on the tail).
In addition, once your eyes adjust to night, you can see plenty from the cockpit.  Starlight, moonlight, and ground-light can give good visibility of terrain, coastlines, clouds, and other things pilots need to see, without any light coming from the airplane.

Some advanced systems use infra-red sensors to see in the dark. The combined image is valuable for seeing terrain and runways when approaching an airport. It can also be used en-route to avoid penetrating convective clouds or entering icing conditions at night.

Tempted to flag this as spam ...

What is a Cirrus?

You can promote your product here. Use following guideline http://aviation.stackexchange.com/help/product-support


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What is ACS on a magneto switch?
I was looking at a magneto switch and the text ACS below attracted my attention. What is its meaning and function?

Where is this photo from?

Related question: why are so many altimeters calibrated for winter conditions? :)

What does the label on the top read?

It's the brand name. ACS is a company that makes aircraft parts, among them ignition switches.